Halogenated solvent remediation

ABSTRACT

Methods for enhancing bioremediation of ground water contaminated with nonaqueous halogenated solvents are disclosed. A preferred method includes adding a composition to the ground water wherein the composition is an electron donor for microbe-mediated reductive dehalogenation of the halogenated solvents and enhances mass transfer of the halogenated solvents from residual source areas into the aqueous phase of the ground water. Illustrative compositions effective in these methods include surfactants such as C 2 —C 4  carboxylic acids and hydroxy acids, salts thereof, esters of C 2 —C 4  carboxylic acids and hydroxy acids, and mixtures thereof. Especially preferred compositions for use in these methods include lactic acid, salts of lactic acid, such as sodium lactate, lactate esters, and mixtures thereof. The microbes are either indigenous to the ground water, or such microbes can be added to the ground water in addition to the composition.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/214,957, filed Jun. 29, 2000, and U.S. ProvisionalApplication No. 60/233,414, filed Sep. 18, 2000.

CONTRACTUAL ORIGIN OF THE INVENTION

[0002] This invention was made with United States Government supportunder Contract No. DE-AC07-99ID13727 awarded by the United StatesDepartment of Energy. The United States Government has certain rights inthe invention.

BACKGROUND OF THE INVENTION

[0003] This invention relates to remediation of environmentalcontamination. More particularly, the invention relates to methods foraccelerating or enhancing in situ dehalogenation of nonaqueoushalogenated solvents in ground water. These methods involve adding tothe contaminated ground water a composition of matter that bothfunctions as an electron donor for halorespiration processes carried outby indigenous or exogenously supplied bacteria, wherein the nonaqueoushalogenated solvents are dehalogenated and degraded to innocuouscompounds, and promotes mass transfer of the nonaqueous halogenatedsolvents from a source into the ground water where such solvents can bebroken down.

[0004] For many years little care was taken in the handling of organicsolvents and other materials that were used in industry and atgovernment installations, such as military bases. Because of poorhandling techniques and, occasionally, intentional dumping, manyindustrial sites and military bases now have contaminated areascontaining relatively high concentrations of these contaminants.Chlorinated solvents, such as trichloroethylene (TCE), perchloroethylene(PCE), and other types of liquids, are common at such sites, and if notremoved can infiltrate groundwater supplies, rendering the water unfitfor consumption and other uses.

[0005] A variety of techniques have been used to promote the removal ofsuch chemical contaminants, both from the soil and from the groundwater. The principle method of ground water remediation currently usedwhere dense, non-aqueous phase liquids (DNAPLs) are involved is what iscommonly referred to as “pump-and-treat” remediation. According to thismethod, wells are drilled into the contaminated area and contaminatedground water is pumped above the surface, where it is treated to removethe contaminants.

[0006] The limitations of the pump-and-treat method have been documentedin articles such as D. M. Mackay & J. A. Cherry, GroundwaterContamination: Pump and Treat Remediation, 23 Environ. Sci. Technol.630-636 (1989). The authors of this article concluded thatpump-and-treat remediation can only be relied on to contain ground watercontamination through the manipulation of hydraulic gradients within anaquifer. The reasons for the failure of the pump-and-treat method todecontaminate aquifers are rooted in the limited aqueous solubility ofmany DNAPLs in ground water and other processes involving contaminantdesorption and diffusion. Because of the low aqueous solubility of mostDNAPLs, their removal by ground water extraction requires exceptionallylong periods of time.

[0007] Due to the general impracticability of the pump-and-treat method,considerable attention has been paid recently to other methods foreffecting remediation. One such process is commonly referred to asenhanced solubilization. This method uses micellar surfactants toincrease the effective solubility of the DNAPLs to accelerate the rateof removal. The mechanism of solubilization displayed by surfactantsarises from the formation of microemulsions by the surfactants, water,and the solubilized DNAPLs. For example, Table 1 shows solubilization ofPCE by various nonionic and anionic surfactants. These data indicatethat even dilute surfactants can significantly increase the aqueoussolubility of PCE. TABLE 1 Surfactant PCE Solubilized SurfactantConcentration (mg/l) Water 0%   240 Nonylphenol ethoxylate 2% 11,700 andits phosphate ester (1:1) Sodium diamyl and dioctyl 4% 85,000sulfosuccinates (1:1) in 500 mg CaCl₂/l Nonylphenol ethoxylate 1%  1,300

[0008] A serious drawback with the surfactant-enhanced aquiferremediation is that the vertical mobility of the solubilized DNAPLssubstantially requires that an aquiclude be present to catch anysolubilized contaminant that migrates vertically. Many aquifers,however, lack such an aquiclude. If the traditional surfactant-enhancedaquifer remediation method were to be used with an aquifer lacking anaquiclude, there is a significant risk that the solubilized DNAPLs willspread vertically and contaminate an increasingly large volume. Anotherdrawback of surfactant-enhanced aquifer remediation is the need to pumphigh concentrations of contaminated water above ground, which results inexposure risks to workers and the environment, and the need to disposeor recycle the surfactant.

[0009] Another method for effecting remediation of ground watercontaminated with DNAPLs is known as enhanced bioremediation. Enhancedbioremediation, as opposed to intrinsic bioremediation, of halogenatedsolvent-contaminated ground water falls into the two broad categories ofaerobic and anaerobic bioremediation. The aerobic processes, regardlessof whether they are carried out in situ or in a bioreactor, requireaddition of (1) oxygen as the electron acceptor for catabolism of thehalogenated solvents, and (2) a carbon source, such as methane, propane,phenol, toluene, or butane. The utilization of an appropriate carbonsource induces an enzyme that fortuitously degrades many halogenatedsolvents, but without any immediate benefit to the microorganismsinvolved. This process has been applied in situ to aqueous contaminationin several instances, and at least one patent has been granted for thisapproach (U.S. Pat. No. 5,384,048). It has also been used to treataqueous contamination in above-ground bioreactors with numerousvariations, especially using proprietary microorganisms and nutrientmixes. Many patents have been granted in this area, e.g., U.S. Pat. No.5,057,221; U.S. Pat. No. 5,962,305; U.S. Pat. No. 5,945,331.

[0010] Anaerobic bioremediation of halogenated solvents is afundamentally different process than aerobic bioremediation. Underappropriate anaerobic conditions, chlorinated solvents can be useddirectly by some microorganisms as electron acceptors through a processthat has come to be known as “chlororespiration,” or, more generally,“halorespiration.” D. L. Freedman & J. M. Gossett, Biological ReductiveDechlorination of Tetrachloroethylene and Trichloroethylene to EthyleneUnder Methanogenic Conditions, 55 Applied Environ. Microbiol. 2144-2155(1989), first published the complete degradation pathway for chlorinatedethenes to ethene. In the following years, several publications reportedevidence that the degradation could be achieved through microbialrespiration, indicating that the microorganisms could actually grow byusing chlorinated solvents directly as electron acceptors. The primaryrequirement to facilitate this process is the addition of a suitableelectron donor or carbon source. Many electron donors have beendescribed in the literature, including acetate, lactate, propionate,butyrate, formate, ethanol, hydrogen, and many others. U.S. Pat. No.5,277,815 issued in 1994 for in situ electron donor addition along withcontrol of redox conditions to effect the desired end products. U.S.Pat. No. 5,578,210 issued later for enhanced anaerobic in situbioremediation using “biotransformation enhancing agents,” i.e.,electron donors such as propylene glycol, glycerol, glutamate, a mixtureof proteose peptone, beef extract, yeast extract, malt extract,dextrose, and ascorbic acid, and mixtures thereof. Based primarily onwhat was publicly available in the scientific literature, studies ofenhanced anaerobic in situ bioremediation of chlorinated solvents beganin the mid-1990s. This approach generally includes electron donoraddition, sometimes with other micronutrients, to facilitatebiotransformation of aqueous-phase contaminants. To date, only a fewlarge-scale studies have been published in the peer-reviewed literature,but environmental consulting companies and remediation contractors areincreasingly using the general approach.

[0011] With one very recent exception, discussed below, all of the workdone in this area to date has focused on the biodegradation of aqueouscontaminants, because microorganisms cannot directly degrade nonaqueouscontaminants. Consequently, bioremediation is not generally thought tobe applicable to sites with residual DNAPLs in the subsurface.Therefore, the technologies currently in use include thermaltechnologies such as steam stripping, in situ chemical oxidation,surfactant flushing, or co-solvent flushing. Surfactant (or co-solvent)flushing, briefly described above, is a chemical process that aims tofacilitate transport of nonaqueous contaminants, but without attentionto biodegradation. At many sites, however, the pump-and-treat processcontinues to be used to hydraulically contain residual source areasalthough it is almost universally accepted that these systems will haveto operate in perpetuity because of their inefficient removal ofnonaqueous contaminants.

[0012] The notable recent exception to the focus of bioremediation onaqueous contaminants away from residual source areas is a study by C. S.Carr et al., Effect of Dechlorinating Bacteria on the Longevity andComposition of PCE-Containing Nonaqueous Phase Liquids under EquilibriumDissolution Conditions, 34 Environ. Sci. Technol. 1088-1094 (2000),demonstrating that anaerobic bioremediation of tetrachloroethene (PCE)enhanced mass transfer from the nonaqueous phase to the aqueous phaseand significantly shortened the longevity of the nonaqueous source. Themechanisms identified were (1) enhanced dissolution of PCE resultingfrom the continuous removal of the compound from the aqueous phase bybacteria, and (2) increased solubility of the intermediate chlorinatedethenes relative to PCE, allowing the total moles of chlorinated ethenesin the aqueous phase to increase due to biotransformation. This study isimportant because it identifies some of the advantages of enhancing masstransfer from the nonaqueous phase to the aqueous phase.

[0013] In view of the foregoing, it will be appreciated that providingmethods for accelerating or enhancing in situ bioremediation ofhalogenated solvents in ground water would be a significant advancementin the art.

BRIEF SUMMARY OF THE INVENTION

[0014] It is an object of the present invention to provide a method forin situ remediation of DNAPLs in ground water wherein capital costs arelow.

[0015] It is also an object of the invention to provide a method for insitu remediation of DNAPLs in ground water wherein mass transfer fromthe nonaqueous phase to the aqueous phase is enhanced.

[0016] It is another object of the invention to provide a method for insitu remediation of DNAPLs in ground water wherein the longevity ofsource areas is shortened.

[0017] It is still another object of the invention to provide a methodfor in situ remediation of DNAPLs in ground water wherein no extractionof contaminated water from the ground is required.

[0018] It is yet another object of the invention to provide a method forin situ remediation of DNAPLs in ground water such that theconcentrations of the solvents are restored to below regulatory limitsand no follow-on remediation activities, other than perhaps monitorednatural attenuation, are needed.

[0019] It is a still further object of the invention to provide a methodfor in situ remediation of DNAPLs in ground water wherein the DNAPLs aremore rapidly removed from the ground water than with prior art methodsand residual source areas are removed.

[0020] It is another object of the invention to provide a method for insitu remediation of DNAPLs in ground water wherein transport isfacilitated and bioavailability of nonaqueous halogenated solvents isenhanced.

[0021] It is still another object of the invention to provide a methodfor in situ remediation of DNAPLs in ground water wherein the method issustainable for low cost and has low maintenance requirements.

[0022] It is yet another object of the invention to provide a method forin situ remediation of DNAPLs in ground water by adding a composition ofmatter that is both an electron donor and a surfactant or enhancer ofmass transfer.

[0023] It is still further an object of the invention to provide amethod for remediation of DNAPLs in ground water wherein destruction ofthe DNAPLs occurs in situ.

[0024] It is a yet further object of the invention to provide a methodfor in situ remediation of DNAPLs in ground water wherein an unobtrusiveappearance is provided and it meets with public acceptance.

[0025] These and other objects can be addressed by providing a methodfor enhancing in situ bioremediation of a nonaqueous halogenated solventin ground water comprising adding to the ground water an amount of anelectron donor sufficient for a halo-respiring microbe in the groundwater to use the nonaqueous halogenated solvent as an electron acceptor,thereby reductively dehalogenating the nonaqueous halogenated solventinto innocuous compounds, wherein the electron donor enhances masstransfer of the nonaqueous halogenated solvents into solution. Theelectron donor preferably functions as a surfactant or co-solvent. Incases where the electron donor is a functional surfactant, it ispreferably added at a concentration above the critical micelleconcentration in water. In cases where the electron donor is afunctional co-solvent, there may be no critical micelle concentration,or if there is a critical micelle concentration in water, the electrondonor is preferably added at a concentration below such critical micelleconcentration. Illustrative electron donors for use in this methodinclude C₂-C₄ carboxylic acids and hydroxy acids, salts thereof, estersof C₂-C₄ carboxylic acids and hydroxy acids, and mixtures thereof. In apreferred embodiment of the invention, the electron donor is a memberselected from the group consisting of lactic acid, salts thereof,lactate esters, and mixtures thereof. Preferred salts of lactic acidinclude sodium lactate, potassium lactate, lithium lactate, ammoniumlactate, calcium lactate, magnesium lactate, manganese lactate, zinclactate, ferrous lactate, aluminum lactate, and mixtures thereof,wherein sodium lactate is especially preferred. Illustrative targets ofthe method include nonaqueous chlorinated solvents, such asperchloroethylene (PCE), trichloroethylene (TCE), dichloroethylene(DCE), vinyl chloride (VC), 1,1,1-trichloroethane (TCA), carbontetrachloride and less chlorinated derivatives thereof, and mixturesthereof. A preferred aspect of the invention relates to enhancing thereductive dehalogenation activity of indigenous halo-respiring microbespresent in the ground water. If halo-respiring microbes are absent orineffective, then such microbes can be exogenously supplied to theground water. Preferably, the microbes are bacteria, such asDehalococcoides ethenogenes strain 195, the Pinellas culture, and thelike, and mixtures thereof. The method degrades the halogenated solventsinto innocuous compounds such as ethylene, ethane, carbon dioxide,water, halogen salts, and mixtures thereof.

[0026] A method for enhancing mass transfer of a nonaqueous halogenatedsolvent present in a nonaqueous residual source of contamination intothe aqueous phase comprises adding to the ground water an effectiveamount of a composition that donates electrons for reductivedehalogenation of the nonaqueous halogenated solvent and functions as asurfactant for solubilizing the nonaqueous halogenated solvent.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a site plan of Test Area North showing the locations ofinjection wells (◯) and monitoring well ().

[0028]FIG. 2 is a cross section of Test Area North showing the locationsand relative depths of injections wells (open bars), monitoring wells(closed bars), and open or screened intervals (hatched bars).

[0029] FIGS. 3A-C show the relationship of electron donor concentrationsand redox conditions to reductive dechlorination at well TAN-31. FIG. 3Ashows COD (solid line) and electron donor (broken line) concentrationsin units of mg/L as a function of time in days. FIG. 3B shows ferrousiron (dotted line), sulfate (solid line), and methane (dashed line)concentrations in units of mg/L as a function of time in days. FIG. 3Cshows TCE, cis-DCE, trans-DCE, VC, and ethene concentrations in units ofμmol/L as a function of time in days.

[0030]FIG. 4 shows facilitated TCE transport and subsequentbiodegradation in well TAN-26.

[0031]FIG. 5 shows surface tension as a function of lactateconcentration for sodium lactate solutions without added ethyl lactate(♦) and with 0.1% ethyl lactate (X), 1% ethyl lactate (⋄), and 10% ethyllactate (*); error bars represent two standard deviations around themean.

[0032]FIG. 6 shows interfacial tension as a function of lactateconcentration for sodium lactate solutions without added ethyl lactate(♦) and with 0.1% ethyl lactate (X), 1% ethyl lactate (⋄), and 10% ethyllactate (*); error bars represent two standard deviations around themean.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Before the present methods for accelerating or enhancing in situbioremediation of halogenated solvents in ground water are disclosed anddescribed, it is to be understood that this invention is not limited tothe particular configurations, process steps, and materials disclosedherein as such configurations, process steps, and materials may varysomewhat. It is also to be understood that the terminology employedherein is used for the purpose of describing particular embodiments onlyand is not intended to be limiting since the scope of the presentinvention will be limited only by the appended claims and equivalentsthereof.

[0034] The publications and other reference materials referred to hereinto describe the background of the invention and to provide additionaldetail regarding its practice are hereby incorporated by reference. Thereferences discussed herein are provided solely for their disclosureprior to the filing date of the present application. Nothing herein isto be construed as an admission that the inventors are not entitled toantedate such disclosure by virtue of prior invention.

[0035] It must be noted that, as used in this specification and theappended claims, the singular forms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to “an electron donor” includes reference to amixture of two or more of such electron donors, reference to “a solvent”includes reference to one or more of such solvents, and reference to “amicrobe” includes reference to a mixture of two or more of suchmicrobes.

[0036] In describing and claiming the present invention, the followingterminology will be used in accordance with the definitions set outbelow.

[0037] As used herein, “comprising,” “including,” “containing,”“characterized by,” and grammatical equivalents thereof are inclusive oropen-ended terms that do not exclude additional, unrecited elements ormethod steps. “Comprising” is to be interpreted as including the morerestrictive terms “consisting of” and “consisting essentially of.”

[0038] As used herein, “consisting of” and grammatical equivalentsthereof exclude any element, step, or ingredient not specified in theclaim.

[0039] As used herein, “consisting essentially of” and grammaticalequivalents thereof limit the scope of a claim to the specifiedmaterials or steps and those that do not materially affect the basic andnovel characteristic or characteristics of the claimed invention.

[0040] As used herein, “PCE,” “perchloroethylene,”“tetrachloroethylene,” and “tetrachloroethene” refer to Cl₂C═CCl₂.

[0041] As used herein, “TCE,” “trichloroethylene,” and “trichloroethene”refer to Cl₂C═CH—Cl.

[0042] As used herein, “DCE,” “dichloroethylene,” and “dichloroethene”refer to Cl—HC═CH—Cl.

[0043] As used herein, “VC” and “vinyl chloride” refer to H₂C═CH—Cl.

[0044] As used herein, “ethylene” and “ethene” refer to H₂C═CH₂.

[0045] As used herein, “chloroethenes” means PCE, TCE, DCE, VC, andmixtures thereof

[0046] As used herein, “biotransformation” means a biological reductionin the number of halogen, e.g., chlorine, atoms covalently bound to anorganic compound. For example, PCE can be biotransformed to TCE, whichcan be biotransformed to DCE, which can be biotransformed to vinylchloride, which can be biotransformed to ethylene. If the rate ofbiotransformation is increased by adding an electron donor to the groundwater, then the biotransformation is enhanced.

[0047] As used herein, “microbe” means a microscopic organism, such asbacteria, protozoa, and some fungi and algae. Bacteria are especiallypreferred microbes according to the present invention. Biotransformationis enhanced, at least in part, by stimulating indigenous, naturallyoccurring microbes in the ground water. If indigenous, naturallyoccurring microbes are not present or are not sufficiently effective,then an appropriate microbe can be added to the ground water, as well asthe electron donor of the present invention. The microbe can be addedbefore, with, or after adding the electron donor to the ground water.Preferably, the microbe is an anaerobic or facultatively anaerobicbacterium. Bacteria known to work within the current processes includeDehalococcoides ethenogenes strain 195 (X. Maymo-Gatell et al.,Isolation of a Bacterium that Reductively DechlorinatesTetrachloroethene to Ethene, 276 Science 1568-1571 (1997)), the Pinellasculture (M.R. Harkness et al., Use of Bioaugmentation To StimulateComplete Reductive Dechlorination of Trichloroethene in Dover SoilColumns, 33 Environmental Sci. Technol. 1100-1109 (1999); D. E. Ellis etal., Bioaugmentation for Accelerated In Situ Anaerobic Bioremediation,34 Environmental Sci. Technol. 2254-2260 (2000)), and the like, andmixtures thereof. Other species, however, are known to function, and thepresent invention is not limited by the examples provided herein.

[0048] As used herein, “surfactant” means a substance that whendissolved in water or an aqueous solution reduces its surface tension orthe interfacial tension between it and another liquid. Surfactants arecharacterized by a structural balance between one or more hydrophilicand hydrophobic groups. This amphiphilic nature causes them to bepreferentially adsorbed at air-water, oil-water, and solid-waterinterfaces, forming oriented monolayers wherein the hydrophilic groupsare in the aqueous phase and the hydrocarbon chains are pointed towardthe air, in contact with the solid surfaces, or immersed in the oilphase. Surfactants are characterized by a critical micelle concentration(cmc), a concentration at which surfactant molecules begin to aggregateinto micelles and above which more micelles are formed. Surfactantsenhance solubility of nonpolar compounds in aqueous solutions byproviding a microenvironment, i.e., the interior of micelles, where thenonpolar compounds can accumulate. In certain preferred embodiments ofthe present invention, the electron donor is a surfactant.

[0049] As used herein, a “co-solvent” is a solvent present in a minoramount as compared to a solvent with which it is mixed. Co-solvents arelike surfactants in that they decrease interfacial tension between twoliquid phases, but they generally do not form micelles. Thus,co-solvents enhance solubility, but not to the extent of surfactants. Inthe context of in situ bioremediation, the rate of enhancedsolubilization mediated by a co-solvent or co-solvents is less likely tooverwhelm the rate of biotransformation. Thus, in certain preferredembodiments of the invention, the electron donor is a co-solvent.

[0050] Chlorinated solvents represent two of the three most commonground water contaminants at hazardous waste sites in the United States,and with their degradation products they account for eight of the top20. Unfortunately, chlorinated solvents are relatively recalcitrantcompounds with low, but toxologically significant, solubilities inwater. Historically, the conventional technology for ground watertreatment has been pump-and-treat methodology. While the pump-and-treatapproach can be useful for achieving hydraulic containment of a groundwater contaminated with chlorinated solvents, it has very rarely beensuccessful for restoration, largely because of the heterogeneity of thesubsurface (i.e., preferential flow paths) and the presence ofnonaqueous phase liquids. This has led to significant research in thelast 10 years on in situ technologies for restoration of ground watercontaminated with chlorinated solvents.

[0051] Residual chlorinated solvent source areas (where nonaqueouscontaminants are present) in the subsurface are especially problematicbecause the combination of low contaminant solubilities and the lack ofmixing in typical ground water flow makes them very long-lived (decadesto centuries). As discussed above, the common perception thatbioremediation cannot effect improvements to the slow mass transfer fromthe nonaqueous to the aqueous phase has limited its applications toaqueous-phase contaminated ground water plumes. Also mentioned above,the technology categories used for these areas other than pump-and-treatinclude thermal technologies such as stream-stripping, in situ chemicaloxidation, surfactant flushing, or co-solvent flushing. While theseapproaches generally result in some rapid mass removal of contaminantsand have worked to varying degrees, they all share a commondisadvantage: they have a high capital cost in the early stages ofremediation. In addition, all except chemical oxidation requireextraction of contaminants from the ground with subsequent treatment.This creates new exposure pathways and increases costs. Finally, thesetechnologies rarely restore ground water to contaminant concentrationsbelow regulatory limits, so follow-on activities are generally required.

[0052] P.V. Roberts et al., Field Study of Organic Water Quality Changesduring Ground Water Recharge in the Palo Alto Baylands, 16 WaterResources Research 1025-1035 (1982), reported one of the first fieldobservations suggesting bioremediation of chloroethenes (PCE, TCE, DCE,and VC). E. J. Bouwer & P. L. McCarty, Transformation of 1- and 2-CarbonHalogenated Aliphatic Organic Compounds under Methanogenic Conditions,45 Applied Environ. Microbiol. 1286-1294 (1983), confirmedbiodegradation of PCE and TCE in the laboratory shortly thereafter. F.Parsons et al., Transformations of Tetrachloroethylene andTrichloroethylene in Microcosms and Groundwater, 76 J. Am. Water WorksAss'n 56-59 (1984), and T. M. Vogel & P. L. McCarty, Biotransformationof Tetrachloroethylene to Trichloroethylene, Dichloroethylene, VinylChloride, and Carbon Dioxide under Methanogenic Conditions, 49 AppliedEnviron. Microbiol. 1080-1083 (1985), demonstrated that DCE and VC weregenerated during biodegradation of PCE under anaerobic conditions.Finally, Freedman and Gossett, supra, reported complete dechlorinationof PCE to ethylene as follows: PCE→ TCE→ DCE→ VC→ ethylene. In each stepof the process the compound was reduced (gaining two electrons) throughsubstitution of a chlorine atom by a hydrogen atom. Hence thisdegradation pathway is often referred to as reductive dechlorination.

[0053] In the reductive dechlorination process, chloroethenes act aselectron acceptors. This implies that the process can be limited in thefield by the availability of sufficient suitable electron donors. Infact, reductive dechlorination also can be totally or partiallyinhibited by the presence of competing inorganic electron acceptors,such as oxygen, nitrate, iron, and sulfate. It is now widely acceptedthat reductive dechlorination occurs to some extent at most field siteswhere chloroethene contamination exists in the presence of a sufficientsupply of electron donors (P. L. McCarty, Biotic and AbioticTransformations of Chlorinated Solvents in Groundwater, in Symposium onNatural Attenuation of Chlorinated Organics in Ground Water 5-9 (Officeof Research and Development, U.S. Environmental Protection Agency,Washington, D.C., EPA/540/R-96/509, 1996); J.M. Gossett & S.H. Zinder,Microbiological Aspects Relevant to Natural Attenuation of ChlorinatedEthenes, in Symposium on Natural Attenuation of Chlorinated Organics inGround Water 10-13 (Office of Research and Development, U.S.Environmental Protection Agency, Washington, D.C., EPA/540/R-96/509,1996); T. H. Wiedemeier et al., Technical Protocol for EvaluatingNatural Attenuation of Chlorinated Solvents in Groundwater,Draft-Revision 1 (Air Force Center for Environmental Excellence,Technology Transfer Division, Brooks Air Force Base, San Antonio, Tex.,1997).

[0054] Many oxidizable organic compounds potentially could make suitableelectron donors. For a potential electron donor to be useful as anamendment for enhanced in situ bioremediation, however, it must be safeto use, facilitate the desired reaction, and be relatively inexpensive.Lactate is a potential electron donor having these properties. It isinnocuous enough for use in the food and medical industries. It has beendemonstrated to facilitate reductive dechlorination of chlorinatedsolvents in several laboratory studies (e.g., W. P. DeBruin et al.,Complete Biological Reductive Transformation of Tetrachloroethylene toEthane, 58 Applied Environ. Microbiol. 1996-2000 (1992); S. A. Gibson &G. W. Sewell, Stimulation of Reductive Dechlorination ofTetrachloroethene in Anaerobic Aquifer Microcosms by Addition ofShort-Chain Organic Acids or Alcohols 1392-1393 (1992), D. E. Fennel etal., Comparison of Butyric Acid, Ethanol, Lactic Acid, and PropionicAcid as Hydrogen Donors for Reductive Dechlorination ofTetrachloroethene, 31 Environ. Sci. Technol. 918-926 (1997). Thecost-effectiveness of lactate has not been thoroughly evaluated, butpreliminary testing suggests that it will be at least as cost-effectiveas other in situ remediation technologies.

[0055] While the use of lactate as an electron donor to facilitatereductive dechlorination is well-established, it has only been appliedfor remediation of aqueous-phase contaminants because of the perceptionthat bioremediation does not significantly enhance mass transfer ofcontaminants from the nonaqueous phase. It is shown herein, however,that the addition of high concentrations of a lactate solution not onlyfacilitates reductive dechlorination of aqueous chloroethenes, but alsosignificantly enhances mass transfer of nonaqueous contaminants, makingthem highly bioavailable. As used herein, “high concentrations” meanshigh relative to the stoichiometric requirement for electron donor todegrade TCE to ethene. Thus, “high concentrations” means about 3-5orders of magnitude greater than such stoichiometric requirements.

[0056] Facilitated transport and enhanced bioavailability of nonaqueouschlorinated solvents through addition of high concentrations of anappropriate electron donor, according to the present invention, takeadvantage of the natural processes that have made natural attenuation sopopular, while also significantly reducing source longevity by enhancingmass transfer to the aqueous phase. The capital costs of the approachare minimal, because only a simple, potentially portable, injectionsystem and monitoring wells are required. Initial mass removal may beslower than some of the other technologies, but it is sustainable for arelatively low cost and requires no extraction of contaminated groundwater except for routine monitoring.

[0057] High concentrations of lactate, for example, not only provide anelectron donor to expedite reductive dechlorination, but also facilitatemass transfer of the nonaqueous chlorinated solvents into the aqueousphase in a manner that makes them highly bioavailable. The lactateappears to act as a surfactant or co-solvent that brings nonaqueouschlorinated solvents into solution. The intimate contact of thechlorinated solvents (electron acceptors) in solution with the lactate(electron donor) enhances bioavailability and leads to rapidbiodegradation. The depletion of the residual contamination source ispotentially greatly accelerated due to the surfactant or co-solventeffect. The use of lactate to facilitate transport of chlorinatedsolvents into the aqueous phase and dramatically increase theirbioavailability opens up a wide range of applications for enhanced insitu bioremediation of chlorinated solvents present as nonaqueous phaseliquids at residual saturation in ground water. The use of a relativelyinexpensive compound that accomplishes the same thing as mildsurfactants or co-solvents, but does not require extraction andabove-ground treatment, combines the advantages of mass removal withthose of enhanced bioremediation.

[0058] All of the advantages of bioremediation, such as low capitalcost, in situ contaminant destruction, unobtrusive appearance, publicacceptance, low maintenance requirements, and the like, can be appliedto residual source areas because, using this process, source longevitycan potentially be greatly reduced. Many of these benefits are enjoyedby owners of contaminated sites, but reduced risk of further releases ofcontaminants to the public and the environment is also important.

[0059] The most appropriate application of this process is to sites withresidual chlorinated solvent source areas in the subsurface, comprisingprimarily nonaqueous contaminants at residual saturation. These arecommon at both federal and industrial facilities. When very large,mobile DNAPL pools are present, mass transfer rates may be too slow toeffect remediation in a reasonable time frame, and more aggressive,capital-intensive approaches may be warranted.

EXAMPLE 1

[0060] A 1-year field evaluation of enhanced in situ bioremediation wasperformed at Test Area North (“TAN”) of the Idaho National Engineeringand Environmental Laboratory. FIG. 1 shows a site plan of TAN, whereinsolid symbols represent monitoring wells (10) and open symbols representinjection wells (12). The locations of a 5,000 μg/L TCE isopleth (14);1,000 μg/L TCE isopleth (16); 100, μg/L TCE isopleth (18); and 5 μg/LTCE isopleth (20) are shown by solid lines. FIG. 2 illustrates a crosssection of this site, showing the surface of the ground (22), anapproximately 63-m (210-feet) fractured basalt unsaturation zone (24)(not drawn to scale), an approximately 60-m (200-feet) fractured basaltaquifer (26), and an impermeable clay interbed (28). The approximatelocation of the TCE secondary source (30) and the 1,000 μg/L TCEisopleth (32) are also indicated. The test was performed to determinewhether this technology has the potential to enhance or replace thedefault pump-and-treat remedy selected for the contaminant source areain the site's Record of Decision. The residual source of chloroethenes(30), primarily TCE with some PCE and DCE, is present in the fracturedbasalt aquifer at the site, about 60 to 120 m below land surface. Theresidual source area (30) is approximately 60 m in diameter, and the TCEplume emanating from the this source is approximately 3 km long. Basedon results of published studies and site-specific laboratory studies (K.S. Sorenson, Design of a Field-Scale Enhanced In Situ BioremediationEvaliuation for Trichloroethene in Ground Water at the Idaho NationalEngineering and Environmental Laboratory, ASAE, St. Joseph, Michigan,Paper No. PNW98-113 (1998)), sodium lactate was chosen as the electrondonor and was injected in Well TSF-05 in concentrations ranging from 3%to 60% by weight (Table 2).

[0061] The initial electron donor addition strategy involved continuousinjection of potable water at 37.85 liters/minute (10 gpm) into WellTSF-05. The electron donor was to be pulsed into this line biweekly. Thepotable water injection began on Nov. 16, 1998, at the beginning of thestartup sampling period. Potable water injection was discontinued onDec. 11, 1998, due to a significant depression of chlorinated etheneconcentrations near the injection well. It was determined that thecontinuous injection of clean water at 37.85 liters/minute (10 gpm)overwhelmed the flux of contaminants from the secondary source. Thiscondition was considered undesirable for evaluation of an in situtechnology, so the electron donor addition strategy was modified suchthat potable water was only injected for 1 hour following injection ofthe electron donor solution to flush the solution into the formationsurrounding the injection well. This was intended to prevent significantquantities of electron donor from collecting in the injection well andto help prevent biofouling.

[0062] The raw electron donor solution used was food grade sodiumlactate. Table 2 presents the injection date, the sodium lactateconcentration in percent by weight, the injection rate in units ofgallons per minute, the total volume of electron donor injected ingallons, and the volume in gallons of potable water injected at 75.7liters/minute (20 gpm) to flush the solution into the formation. Lactateinjections began on Jan. 7, 1999, and were continued until Sep. 8, 1999.Four injection solution concentrations were used, each being more dilutethan the previous solution. The dilutions were made in an effort to keepthe lactate in the upper part of the aquifer, reducing density effectsthat cause the electron donor solution to sink to the base of theaquifer. Because the total mass of lactate was kept constant, and theinjection flow rate was not dramatically increased, the duration ofinjection increased from 30 minutes to 4 hours. TABLE 2 Sodium InjectionTotal Lactate Flow Volume Potable Water Concentration Rate InjectedFlush Volume Date (%) (gpm) (gal) (gal) 1/7/1999 60  10   300 1,2001/12/1999 60  10   300 1,200 1/19/1999 60  10   300 1,200 2/2/1999 30 20   600 1,200 2/9/1999 30  20   600 1,200 2/16/1999 30  20   600 1,2002/23/1999 30  20   600 1,200 3/2/1999 6 25 1,500 1,200 3/4/1999 6 251,500 1,200 3/9/1999 6 25 1,500 1,200 3/11/1999 6 25 1,500 1,2003/16/1999 6 25 1,500 1,200 3/18/1999 6 25 1,500 1,200 3/23/1999 6 251,500 1,200 3/25/1999 6 25 1,500 1,200 3/30/1999 6 25 1,500 1,2004/1/1999 6 25 1,500 1,200 4/6/1999 6 25 1,500 1,200 4/8/1999 6 25 1,5001,200 4/13/1999 6 25 1,500 1,200 4/15/1999 6 25 1,500 1,200 4/22/1999 625 3,000 1,200 4/28/1999 6 25 3,000 1,200 5/5/1999 6 25 3,000 1,2005/12/1999 6 25 3,000 1,200 5/19/1999 6 25 3,000 1,200 5/26/1999 6 253,000 1,200 6/2/1999 6 25 3,000 1,200 6/9/1999 3 25 6,000 1,2006/16/1999 3 25 6,000 1,200 6/23/1999 3 25 6,000 1,200 6/30/1999 3 256,000 1,200 7/7/1999 3 25 6,000 1,200 7/14/1999 3 25 6,000 1,2007/21/1999 3 25 6,000 1,200 7/28/1999 3 25 6,000 1,200 8/4/1999 3 256,000 1,200 8/11/1999 3 25 6,000 1,200 8/18/1999 3 25 6,000 1,2008/25/1999 3 25 6,000 1,200 9/1/1999 3 25 6,000 1,200 9/8/1999 3 25 6,0001,200

[0063] Eleven monitoring wells (i.e., TAN-D2, TAN-9, TAN-10A, TAN-25,TAN-26, TAN-27, TAN-28, TAN-29, TAN-30A, TAN-31, and TAN-37) weresampled biweekly and analyzed for electron donors, biological activityindicators, competing inorganic electron acceptors and their reducedproducts, chloroethenes, ethene, pH, temperature, and specificconductivity.

[0064] Electron Donor Distribution. Because concentrated lactatesolutions are denser than water, their injection into an aquifer cancause density-driven flow downward in the aquifer. At TAN, somedensity-driven flow was desirable during lactate addition because thezone to be treated was approximately 60 m thick but the injection well(TSF-05) was completed only in the upper 30 m. It was apparent after thefirst month of injections, however, that too much of the lactatesolution was moving into the lower half of the zone before spreadinghorizontally in the upper half of the zone. For this reason, theconcentration of the lactate was reduced and the injection duration wasincreased in steps over several months. The importance of the lactateaddition strategy can be seen in well TAN-31, a cross-gradient wellcompleted in the upper half of the treatment zone approximately 15 mfrom the injection well (FIG. 3A). The increasing lactate concentrationsafter 150 days correspond to the third (and final) step in changing theinjection strategy.

[0065] Redox Conditions and Reductive Dechlorination. The effect oflactate addition on redox conditions, and ultimately on reductivedechlorination, is evident in FIGS. 3A-C. Sulfate reduction actuallybegan at the fairly modest lactate concentrations in well TAN-31 duringthe first 100 days of the test, with minor iron reduction evident fromincreasing ferrous iron concentrations (FIG. 3B). After sulfate wasdepleted, TCE transformation to cis-1,2-dichloroethene (cis-DCE) began(FIG. 3C). Reductive dechlorination stopped at cis-DCE until the lactateconcentrations increased after 150 days and methanogenesis began.Transformation of cis-DCE to vinyl chloride and ethene coincided almostexactly with the onset of methanogenesis. Beyond about 200 days from thestart of the test, ethene was by far the largest constituent at thissampling location.

[0066] Enhanced reductive dechlorination of TCE to ethene was observedin all wells receiving significant lactate concentrations.

[0067] Based on the results of the field evaluation, enhanced in situbioremediation was selected to replace pump-and-treat for remediation ofthe residual contaminant source area at Test Area North. Of particularimportance in the decision process was the fact that the process waseffective not only for degrading chlorinated solvents in the aqueousphase, but also that the process seemed to have a significant impact onthe residual source itself.

[0068] Enhanced Bioavailabilitv. A surprising observation during thefield evaluation was a dramatic increase in TCE concentrations deep inthe aquifer soon after sodium lactate addition began (FIG. 4). The TCEincrease appeared to occur essentially simultaneously with the arrivalof the highly concentrated electron donor solution. In addition, thepeak TCE concentration was actually significantly higher than historicalmeasurements for well TAN-26. These observations strongly suggest thatthe transport of TCE to well TAN-26 was associated with the downwardmigration of the electron donor. This could occur through twomechanisms. One possible explanation for the large, rapid increase inTCE concentrations is that the lactate solution simply pushed secondarysource material along in front of it as it migrated out from wellTAN-05, through the secondary source, and down toward well TAN-26.However, tritium was a co-contaminant in the residual source material,and consideration of the tritium data in well TAN-26 appears to rule outthis possibility. In fact, tritium concentrations were completelyunaffected in spite of large increases in organic contaminantconcentrations (TCE and DCE).

[0069] A second possible explanation for increased TCE concentrations inwell TAN-26 is that the lactate injection led to facilitated transportof the organic contaminants. Three hypotheses that could explainfacilitated transport are as follows: (1) that the lactate solution actsas a co-solvent for the organic contaminants, (2) that the lactate actsas a surfactant, and (3) that the lactate solution, because of its highconcentration, displaces sorbed chlorinated ethenes, driving them intosolution. All of these mechanisms would result in facilitated transportof the chlorinated ethenes in intimate contact with the lactate solutionand would make more of the chlorinated ethenes bioavailable. Thebehavior of the TCE in well TAN-26 after the peak concentration suggeststhat it was, in fact, extremely bioavailable. The drop in TCEconcentration from the peak concentration to undetectable levelsoccurred with a TCE half-life of less than 20 days (assuming first-orderkinetics for illustration). Just as important for the facilitatedtransport hypothesis, cis-DCE increased to a peak concentration within20% of the peak TCE concentration (indicating an excellent massbalance), and then remained elevated near that peak concentration. Thesignificance of this point is that the lactate injection was continuing,so if the hypothesis were valid it would be expected to continuebringing the organic contaminants with it as it migrated through thesecondary source. After biological activity increased, the TCE wastransformed to cis-DCE before reaching well TAN-26, but as shown in FIG.4, the total ethene level remained approximately constant. After severalmonths the total ethene concentration dropped, but this was expected(and intentional) because the lactate solution concentration had beenreduced by a factor of 20 in June. This change reduced the density ofthe solution significantly, so less lactate, and therefore less totalethenes, was transported to well TAN-26. Thus, the concentrationdecrease supports the hypothesis of facilitated transport.

[0070] The facilitated transport makes available for reductivedechlorination large quantities of the chlorinated ethenes thatotherwise would remain associated with the secondary source. As shown bythe well TAN-26 data, once made available by the lactate solution, theTCE was, in fact, rapidly degraded. Enhanced bioavailability ofchlorinated ethenes in the secondary source would greatly decrease thelongevity of the source.

EXAMPLE 2

[0071] Based on the field results presented in Example 1, laboratorystudies were preformed to confirm that the enhanced bioavailability ofTCE observed in the field was due to co-solvent or surfactant behaviorresulting from the use of high concentrations of sodium lactate. Twofundamental properties used to screen the co-solvent or surfactantproperties of a solution are surface tension and interfacial tension.Surface tension measures the force per unit length along the interfacebetween a liquid and air due to its tension. When a co-solvent orsurfactant is present in an aqueous liquid at increasing concentrations,the surface tension of that liquid decreases. Interfacial tension issimilar to surface tension except that it measures the force per unitlength along the interface between two liquid phases arising from thesurface free energy. The higher the interfacial tension between twoliquids, the less likely one is to dissolve into the other, and the moredifficult it is for one to be transported within the other. Thus,perhaps the most significant property of co-solvents and surfactants inthe context of chlorinated solvent remediation is that they decrease theinterfacial tension between the aqueous phase (groundwater) and theorganic nonaqueous phase so that the solubility (or mobility fororder-of-magnitude decreases) of the nonaqueous phase is enhanced.

[0072] The laboratory study performed to confirm the co-solventproperties of the high concentration electron donor solution measuredthe surface tension of electron donor solutions at variousconcentrations. Next, interfacial tensions between the same electrondonor solutions and nonaqueous phase TCE were measured. Two types ofelectron donor solutions were used. The first was differentconcentrations of sodium lactate, the electron donor used in Example 1.The second was various mixtures of sodium lactate and ethyl lactate.Ethyl lactate was chosen because it is a lactate-based compound that isused in some industries as a solvent. Thus it was believed ethyl lactatemight further enhance the co-solvent behavior observed, while stillacting as a suitable electron donor for bioremediation. It is believedthat mixtures of sodium lactate and ethyl lactate have never before beenused for bioremediation. Surface and interfacial tension measurementswere made using the pendant drop method (M. J. Rosen, ed.,Structure/Performance Relationships in Surfactants, American ChemicalSociety, Washington D.C. 329 (1984); R. D. Bagnall & P. A. Arundel, TheProfile Area of Pendant Drops, 82 J. Phys. Chem. 898 (1978)) coupledwith real-time video imaging (M.D. Herd et al., Interfacial Tensions ofMicrobial Surfactants Determined by Real-Time Video Imaging of PendantDrops, Proceedings paper number SPE/DOE 24206 513-519, SPE/DOE EighthSymposium on Enhanced Oil Recovery, Tulsa, Okla. (1992)).

[0073] The results of the surface tension experiment are shown in FIG.5. Surface tension is plotted on the vertical axis, while sodium lactateconcentration for each solution is plotted on the horizontal axis. Thedifferent lines on the plot are for different concentrations of ethyllactate ranging from 0 to 10% mixed with the sodium lactate solution.Error bars represent two standard deviations around the mean.Examination of the 0% ethyl lactate line (sodium lactate only) revealsthat at sodium lactate concentrations from 0.01 to 7%, almost no changein surface tension occurred. As the concentration was increased to 30and 60%, however, a dramatic decrease in the surface tension wasmeasured. This result confirms that sodium lactate begins to exhibitco-solvent properties at high concentrations. These concentrations areabout 3 orders of magnitude higher than reported in other studies, whichexplains the surprising results discussed in Example 1.

[0074] In an effort to decrease the sodium lactate concentrationsrequired to lower the surface tension of the solution, mixtures withethyl lactate were evaluated. As seen in FIG. 5, the addition of 1% and10% ethyl lactate to the different sodium lactate solutions had apronounced effect on the solution's surface tension. Thus, the additionof ethyl lactate to the sodium lactate electron donor solution enhancesits co-solvent properties. The choice of optimum concentration would bea matter of design for a specific remediation. If only slightly enhancedbioavailability of the solvents were desired, the high concentrationsodium lactate solution would be appropriate. If a large degree ofenhanced bioavailability were desired, the addition of 1 to 10% ethyllactate would be appropriate.

[0075] The results of the interfacial tension measurements are shown inFIG. 6. As before, error bars represent two standard deviations aroundthe mean. For 0% ethyl lactate (sodium lactate only), the effect ofincreasing sodium lactate concentration occurs at lower concentrationsfor interfacial tension than observed in the surface tensionmeasurements. Interfacial tension decreased by about 26% when sodiumlactate was increased from 0.1 to 3% (still 2 orders of magnitude aboveprevious studies). When sodium lactate was increased to 30%, theinterfacial tension was decreased to 47% of the value at a sodiumlactate concentration of 0.1%. Again, the importance of high sodiumlactate concentrations for achieving the co-solvent properties isapparent.

[0076] As ethyl lactate was added to the sodium lactate solutions, it isclear that the ethyl lactate concentration is the primary factoraffecting interfacial tension. FIG. 6 shows that the interfacial tensionbecomes relatively insensitive to sodium lactate concentration for theethyl lactate mixtures. From a remediation design standpoint, thissimplifies things because co-solvent effects appear to be affected byonly one component of the mixture. Interestingly, only the 10% ethyllactate mixture displayed lower surface tensions than the 30% sodiumlactate solution with no ethyl lactate.

I claim:
 1. A method for enhancing in situ bioremediation of anonaqueous halogenated solvent in ground water comprising adding to theground water an amount of an electron donor sufficient for ahalo-respiring microbe in the ground water to use the nonaqueoushalogenated solvent as an electron acceptor, thereby reductivelydehalogenating the nonaqueous halogenated solvent into innocuouscompounds, wherein said electron donor enhances mass transfer of thenonaqueous halogenated solvents into solution.
 2. The method of claim 1wherein said electron donor functions as a surfactant.
 3. The method ofclaim 2 wherein said electron donor is added at a concentration abovethe critical micelle concentration in water.
 4. The method of claim 1wherein said electron donor functions as a co-solvent.
 5. The method ofclaim 1 wherein said electron donor is a member selected from the groupconsisting of C₂-C₄ carboxylic acids and hydroxy acids, salts thereof,esters of C₂-C₄ carboxylic acids and hydroxy acids, and mixturesthereof.
 6. The method of claim 5 wherein said electron donor is amember selected from the group consisting of lactic acid, salts thereof,lactate esters, and mixtures thereof.
 7. The method of claim 6 whereinsaid salts of lactic acid are selected from the group consisting ofsodium lactate, potassium lactate, lithium lactate, ammonium lactate,calcium lactate, magnesium lactate, manganese lactate, zinc lactate,ferrous lactate, aluminum lactate, and mixtures thereof.
 8. The methodof claim 7 wherein said electron donor is a mixture of sodium lactateand ethyl lactate.
 9. The method of claim 1 wherein said electron donorcomprises sodium lactate.
 10. The method of claim 1 wherein saidelectron donor comprises ethyl lactate.
 11. The method of claim 10wherein said electron donor comprises a mixture of sodium lactate andethyl lactate.
 12. The method of claim 1 wherein said nonaqueoushalogenated solvent comprises a nonaqueous chlorinated solvent.
 13. Themethod of claim 12 wherein said nonaqueous chlorinated solvent is amember selected from the group consisting of perchloroethylene (PCE),trichloroethylene (TCE), dichloroethylene (DCE), vinyl chloride (VC),and mixtures thereof.
 14. The method of claim 1 wherein said microbe isindigenous to the ground water.
 15. The method of claim 1 furthercomprising adding the halo-respiring microbe to the ground water. 16.The method of claim 15 wherein said halo-respiring microbe is achloro-respiring microbe.
 17. The method of claim 16 wherein saidchloro-respiring microbe is a bacterium.
 18. The method of claim 17wherein said bacterium is a member selected from the group consisting ofDehalococcoides ethenogenes strain 195, the Pinellas culture, andmixtures thereof.
 19. The method of claim 1 wherein said innocuouscompounds are members selected from the group consisting of ethylene,ethane, carbon dioxide, water, halogen salts, and mixtures thereof. 20.A method for enhancing bioremediation of a nonaqueous chlorinatedsolvent in ground water comprising adding to the ground water an amountof an electron donor sufficient for a chloro-respiring microbe to usethe nonaqueous chlorinated solvent as an electron acceptor, thusreductively dechlorinating the nonaqueous chlorinated solvent intoinnocuous compounds, wherein said electron donor enhances mass transferof the nonaqueous chlorinated solvents into solution.
 21. The method ofclaim 20 wherein said electron donor functions as a surfactant.
 22. Themethod of claim 21 wherein said electron donor is added at aconcentration above the critical micelle concentration in water.
 23. Themethod of claim 20 wherein said electron donor functions as aco-solvent.
 24. The method of claim 20 wherein said electron donor is amember selected from the group consisting of C₂-C₄ carboxylic acids andhydroxy acids, salts thereof, esters of C₂-C₄ carboxylic acids andhydroxy acids, and mixtures thereof.
 25. The method of claim 24 whereinsaid electron donor is a member selected from the group consisting oflactic acid, salts thereof, lactate esters, and mixtures thereof. 26.The method of claim 25 wherein said salts of lactic acid are selectedfrom the group consisting of sodium lactate, potassium lactate, lithiumlactate, ammonium lactate, calcium lactate, magnesium lactate, manganeselactate, zinc lactate, ferrous lactate, aluminum lactate, and mixturesthereof.
 27. The method of claim 20 wherein said electron donorcomprises sodium lactate.
 28. The method of claim 20 wherein saidelectron donor comprises ethyl lactate.
 29. The method of claim 28wherein said electron donor comprises a mixture of sodium lactate andethyl lactate.
 30. The method of claim 20 wherein said nonaqueouschlorinated solvent is a member selected from the group consisting ofperchloroethylene (PCE), trichloroethylene (TCE), dichloroethylene(DCE), vinyl chloride (VC), and mixtures thereof.
 31. The method ofclaim 20 wherein said microbe is indigenous to the ground water.
 32. Themethod of claim 20 further comprising adding the chloro-respiringmicrobe to the ground water.
 33. The method of claim 32 wherein saidchloro-respiring microbe is a bacterium.
 34. The method of claim 33wherein said bacterium is a member selected from the group consisting ofDehalococcoides ethenogenes strain 195, the Pinellas culture, andmixtures thereof.
 35. The method of claim 20 wherein said innocuouscompounds are members selected from the group consisting of ethylene,ethane, carbon dioxide, water, chlorine salts, and mixtures thereof. 36.A method for enhancing mass transfer of a nonaqueous halogenated solventpresent in a nonaqueous residual source of contamination in groundwater, said ground water comprising an aqueous phase, into said aqueousphase comprising adding to said ground water an effective amount of acomposition that donates electrons for microbe-mediated reductivedehalogenation of said nonaqueous halogenated solvent into innocuouscompounds and functions as a surfactant or co-solvent for solubilizingsaid nonaqueous halogenated solvent.
 37. The method of claim 36 whereinsaid composition functions as a surfactant and is added at aconcentration above the critical micelle concentration in water.
 38. Themethod of claim 36 wherein said composition functions as a co-solvent.39. The method of claim 36 wherein said composition is a member selectedfrom the group consisting of C₂-C₄ carboxylic acids and hydroxy acids,salts thereof, esters of C₂-C₄ carboxylic acids and hydroxy acids, andmixtures thereof.
 40. The method of claim 39 wherein said composition isa member selected from the group consisting of lactic acid, saltsthereof, lactate esters, and mixtures thereof.
 41. The method of claim40 wherein said salts of lactic acid are selected from the groupconsisting of sodium lactate, potassium lactate, lithium lactate,ammonium lactate, calcium lactate, magnesium lactate, manganese lactate,zinc lactate, ferrous lactate, aluminum lactate, and mixtures thereof.42. The method of claim 36 wherein said composition comprises sodiumlactate.
 43. The method of claim 36 wherein said composition comprisesethyl lactate.
 44. The method of claim 43 wherein said compositioncomprises a mixture of sodium lactate and ethyl lactate.
 45. The methodof claim 36 wherein said nonaqueous halogenated solvent is a memberselected from the group consisting of perchloroethylene (PCE),trichioroethylene (TCE), dichloroethylene (DCE), vinyl chloride (VC),and mixtures thereof.
 46. The method of claim 36 wherein said microbe isindigenous to the ground water.
 47. The method of claim 36 furthercomprising adding a chloro-respiring microbe to the ground water formediating said reductive dehalogenation.
 48. The method of claim 47wherein said chloro-respiring microbe is a bacterium.
 49. The method ofclaim 48 wherein said bacterium is a member selected from the groupconsisting of Dehalococcoides ethenogenes strain 195, the Pinellasculture, and mixtures thereof.
 50. The method of claim 36 wherein saidinnocuous compounds are members selected from the group consisting ofethylene, ethane, carbon dioxide, water, chlorine salts, and mixturesthereof.